Well no 2 programs will ever be 100% accurate at proving stability - but Prime95 and LinX combined are a damn good start.

To start, a LinX run with at least 50% of your RAM for an hour should get you a baseline of stability. After you pass that, run Prime95 small FFTs for 4-8 hours (your preference, but I usually run 8 hours). If you can pass both of those tests you're on your way to having a stable system. I've yet to put a system together that could pass those two tests combined and move on to fail something else.